node-editor: Save test data relative to current working directory

node-editor: Save test data relative to current working directory

This avoids hard-coding the GTK source directory into the binary, which
would make the binary non-reproducible across different source paths,
particularly in a distro build environment where the source directory
used by autobuilders will often not exist on end-user systems. The node
editor can still be used to create new test-cases by running it with the
GTK source directory as its current working directory.

Bug: https://gitlab.gnome.org/GNOME/gtk/-/issues/5403
Forwarded: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/5297
Signed-off-by: Simon McVittie <smcv@debian.org>
Gbp-Pq: Name node-editor-Save-test-data-relative-to-current-working-di.patch